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Raft puts down roots in London flats

SUBSIDENCE AFFECTING three blocks of flats founded on London Clay has seen a substantial underpinning project in north London.

The work was needed after tree root growth caused dehydration over a period of years.

Abbey Pynford has been working on the design and build contract for Quinn (London) and suggested a reinforced concrete piled raft structure installed on 160mm clayboard for future heave protection.

The buildings posed difficulties for underpinning with poor construction tolerances creating eccentric wall loads for propping. Substantial traditional mass concrete foundation projections also affected pile positions.

These were offset up to 1m from the internal face of the brickwork creating substantial bending moments in an up to 675mm raft slab. The cantilevering slab also created high pile loads greater than 330kN.

Traditional mass concrete underpinning was ruled out due to the depth of excavations required to reach depths unaffected by tree root action.

A 5t hydraulic crawler mounted drilling rig install 300mm diameter open auger bored piles to depths up to 16m. These were sleeved for heave protection to a 3m depth below piling platform level.

Propping was then installed to support the superstructure, capable of withstanding loads up to 260kN/m before building an FEA designed raft slabs. Once the slab reaches an adequate compressive strength the props will be removed.

Work began on the original value £240,000 contract in October and is due for completion by the middle of this month.

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